Soil N index enhancement by organic fertilizer application depends on aggregate size

Nitrogen (N) is the major limiting nutrient determining yield and quality of crops. Many factors related to the N cycle jointly determine its overall functioning and plant uptake. However, the effects of fertilizer types on the soil N cycle in different soil aggregate fractions remain elusive. We developed a soil N index consisting of N substrates, N drivers and N processes to more systematically understand the overall soil N cycle. We used the N index to evaluate the effects of fertilizer types (organic vs. mineral) and application rates in three fractions of soil aggregates. Compared to mineral fertilizer application, organic fertilizer application increased the soil N index by 0.41–0.17 in microaggregates, but not in large and small macroaggregates. Higher rates of organic or mineral fertilizer application also increased the soil N index by 0.50–0.74 in microaggregates but not in large and small macroaggregates. In the low mineral fertilizer treatment, the soil N index in large and small macroaggregates was respectively 0.52 and 0.74 higher than in microaggregates. We conclude that organic fertilizer application and high fertilizer application rates boost the soil N index, which depends on aggregate size. Our study indicates that examining soil within microaggregates may be crucial for understanding functional changes in the soil N cycle.